1. Field of the Invention
The present invention relates to a looped bus system, or more in particular to a looped bus system having functionally-separate starting and terminating points and suitable for communication between a plurality of card-like circuit members in a single housing, and to a method of construction thereof.
2. Description of the Related Art
In a public communication network, what is called a switching system is used which is operated for selecting an optimum route in accordance with the address of transmitted information in order to transmit the information to a given address and apply the information to a selected output port.
The switching system includes a plurality of pairs of input-output ports. The function is essential for transferring information from an arbitrary interface card accommodating an input port to a given interface card accommodating an output port. In order to secure communication between given input and output ports, the conventional switching system includes a space switch having hardware configured in a matrix and a time switch for selecting an arbitrary destination by changing the temporal position.
It is also possible to realize an information exchange function by the input interface card and the output interface card accessing the shared medium. The medium shared by a plurality of cards for the purpose of communication is generally called a "bus".
In "Distributed Queue Dual Bus (DQDB) Subnetwork of a Metropolitan Area Network", IEEE 802.6 (December 1990) proposed by IEEE engaged in preparing international standards, a special shape of bus is made thereby to realize an additional function.
FIGS. 2 and 3 are diagrams showing the conventional bus configuration described in the aforementioned reference. FIG. 2 shows the bus state under normal condition. In this specification, elements 10-1 to 10-6, providing a source and a destination of information transmitted through the bus, are called "nodes". Each node is connected to a pair of buses 20a, 20b making up a shared medium. Each bus is a unidirectional one with a predetermined signal transmission direction. In order to permit communication between given nodes, the nodes are connected by two buses having opposite directions of signal transmission.
In the description that follows, one of the buses in pair is called "bus A" 20a, and the other "bus B" 20b for the sake of convenience. Each bus has a starting point and a terminal, called "head of bus" and "terminal of bus" hereinafter respectively.
Also, the node having a head of bus is called "head of bus A" or "head of bus B". Each head of bus has a head of bus function 30a or 30b, and each terminal of bus includes a termination of bus function 31a or 31b. The head of bus function and the termination of bus function will be described in more detail later.
FIG. 2 shows a case in which the head of bus function and the termination of bus function of the buses 20a, 20b exist in the same node 10-1.
As shown in FIG. 2, the bus form with the bus configured in loop is generally called the looped bus, in which transfer information is discarded at the terminal of bus thereby preventing the same information from going round the same bus.
FIG. 3 shows the state of a bus in which a failure condition 40, caused by a defect of the medium, etc., occurred in a part of the bus.
In the case where a failure condition has occurred between the nodes 10-4 and 10-5, for example, the head of bus and the terminal of bus for the node 10-1 that have thus far been separated from each other come to be connected, and the head of bus function and the termination of bus function are transferred to another node. In this way, communication is made possible among all the nodes.
For example, the node 10-5, one of the nodes adjacent to the point of failure, is used to perform the function 30a of head of bus A and the function 31b of termination of bus B. Also, the function 31a of terminal of bus A and the function 30b of head of bus B are performed by the other node 10-4 adjacent to the point of failure. In this way, the transfer of the head and termination of bus under failure condition is called "bus reconfiguration", whereby the communication sharing the bus A 20a and the bus B 20b is made possible between arbitrary nodes.
According to the looped bus system described in the above-mentioned reference, each node is an independent device, and the looped bus is used as a transmission line connecting the devices. In a looped bus system, the conversion operation of information type can be simplified or eliminated by making the type of transfer information on the bus coincide with the type of communication information handled by each node.
In a system used for a Broadband ISDN (Integrated Services Digital Network), for example, information is transmitted in packets of fixed length called the ATM (Asynchronous Transfer Mode) cells, as shown in FIG. 4. In the case where the above-mentioned looped bus is employed for the system making up a part of the Broadband ISDN, therefore, the information flowing on the looped bus is desirably of a type of fixed-length packet similar to the ATM cell.
FIG. 4 shows a type of ATM cell. The ATM cell 5 is a fixed-length packet of 53 bytes, for instance, and includes a 5-byte header 50 and a 48-byte user information 53. The header 50 has a 4-byte header information 51 and one-byte header check code 52.
When information of the ATM cell type described above is transmitted on a looped bus, the bus access must be controllable in time units corresponding to the length of the ATM cell if the bus is to be used efficiently. The time unit on the bus required for sending out each ATM cell is called the "slot". By generating information representing the time unit or position of each slot at the head of a looped bus, all the nodes connected with the particular bus can be supplied with the information on the time or position where the cell is to be written. In other words, the primary role of the head of bus function is to set a slot, which is the reason why the head of bus function is called the "slot generator" in the above-mentioned reference.
A time chart of signals flowing on the bus is shown in FIG. 5.
When slots 60-i, 60-(i+1), and so on are set at the head of bus (0), for example, these slots are sequentially transmitted to nodes 10-1, 10-2 and so on through the bus. The node 10-1, as shown in (1), for example, outputs ATM cells 70-1, 70-2 in specified or arbitrary slots. In similar fashion, the node 10-2, as shown in (2), outputs an ATM cell 71-1 to an empty slot.
Nodes successively send out cells in this way, and the output of the last node 10-n makes up an ATM cell train as shown in (n), and the resulting output (n+1) flows into the bus terminal. One of the termination of bus functions is to monitor the frequency of ATM cells in or the frequency of bus utilization by the cell train.
FIGS. 6 to 9 show a node configuration applied to the looped bus system of FIGS. 2 and 3.
Each node 10-i includes a physical interface function 103-i for accomplishing the interface with the bus and a media access control function 104-i for making access to the bus.
The media access control function 104-i has an access control function 100-i for controlling the bus access and bus control functions 102A-i and 102B-i. The head of bus functions 30a, 30b and the termination of bus functions 31a, 31b described above are included in the bus control function 102A-i or 102B-i.
The nodes located midway of the looped bus, i.e., those nodes not requiring the head of bus function and the termination of bus function, are operated to allow the bus control functions 102A-i and 102B-i to pass the received information, as shown in FIG. 6.
The nodes, like the node 10-1 in FIG. 2, providing the head and terminal of the bus A and the head and terminal of the bus B, are such that as shown in FIG. 7, the head of bus functions 30a, 30b and the termination of bus functions 31a, 31b in the bus control function 102A-i are active. In this case, the buses A and B are disconnected in the bus control function 102A-i, so that information is not transmitted from termination of bus function 31a to head of bus function 30a or from termination of bus function 31b to head of bus function 30a.
The node, like the node 10-5 in FIG. 3, which provides the head of bus A and the terminal of bus B adjacent to a point of failure is in such a condition that, as shown in FIG. 8, the head of bus function 30a and the termination of bus function 31b in the bus control function 102A-i are active.
Also, the node, like the node 10-4 in FIG. 3, which accommodates the head of bus B and the terminal of bus A adjacent to a point of failure, assumes a condition in which, as shown in FIG. 9, the head of bus function 30b and the termination of bus function 31a are active in the bus control function 102B-i.
In the aforementioned conventional looped bus system, the node 10-1 leading the looped bus under normal condition requires the head of bus A function 30a and the head of bus B function 30b at the same time. For this reason, the bus control function 102A-1 must be provided with the head of bus functions 30a and 30b as shown in FIG. 7.
Further, on the assumption that a bus failure has occurred between the node 10-1 and the adjacent node 10-2, for example, the node 10-1 is required to have the head of bus function 30b with the bus control function 102B-1. In other words, unlike the remaining buses requiring only two head of bus functions, the leading node 10-1 of the looped bus under normal condition requires three head of bus functions, thereby giving rise to the problem that a plurality of nodes having different structures are required for configuration of a looped bus system.
Furthermore, in the above-mentioned conventional looped bus system under normal condition, the heads of the bus A 20a and of the bus B 20b are arranged in the same node, so that upon occurrence of a bus failure, the heads of the bus A 20a and bus B 20b are arranged in different nodes determined by the position of failure. The resulting problem is that the operation for what is called "revertive switchover" is necessary when returning the bus to normal operating condition after the cause of bus failure is removed, which leads to a time loss.